Combustion engines, such as, diesel or gasoline engines, often employ emission control systems that are designed to reduce the emission of pollutants from the engine. For example, some combustion engines may utilize exhaust gas recirculation (EGR) systems for reducing nitrogen oxide (NOx) emissions. The EGR systems are configured to recirculate cooled engine exhaust gas back into the combustion chamber of the engine to reduce the volume of air and/or other combustible matter in the cylinder or the pre-combustion mixture. By replacing a portion of the oxygen or combustible material present during the combustion process with cooled, spent exhaust gas, EGR systems may reduce the temperatures obtained during combustion process and thereby reduce the amount of NOx that is generated by the combustion.
Before the EGR system recirculates exhaust gas into the combustion chamber, the exhaust gas is typically cooled by an EGR cooler or heat exchanger. A coolant, such as antifreeze mixtures or non-aqueous solutions, among others, typically circulates through the EGR cooler. According to some designs, the coolant and/or the heated exhaust gases flow through tubes, a jacket, or other forms of conduits in the EGR cooler. The EGR cooler may be configured so that heated exhaust gases flow around and/or over tubes containing flowing coolant, or vice versa, causing heat from the exhaust gas to be transferred to the coolant. The EGR cooler may also include fins that assist with the transfer of heat from the exhaust gas to the coolant.
While the temperature of the coolant is intended to increase as the coolant absorbs heat from the exhaust gas in the EGR cooler, the EGR cooler, as well as other EGR system components, may be damaged if the coolant exceeds desired temperatures, such as when the coolant reaches or exceeds its boiling temperature. Complicating matters is the fact that the temperature at which the coolant boils changes as the coolant's pressure in the EGR cooler changes.
Undesirable fluctuations in coolant temperature, such as temperature fluctuations above and below boiling temperatures, may cause thermal fatigue within the EGR cooler. Thermal fatigue may damage the EGR cooler, including, for example, cracking of the tubes or jackets that transport the coolant in the EGR cooler, among other EGR cooler components. To avoid these problems, EGR coolers are often oversized. Using oversized components takes up valuable space in the engine compartment and also increases manufacturing costs and component weight. Another approach is to increase rate of coolant flow in the EGR cooler. However, increasing the coolant flow rate may cause or expedite erosion in the tubes or other portions of the EGR cooler that transport the coolant. Further, it typically becomes increasingly difficult to continually oversize the EGR cooler and/or increase coolant flow rate as emission requirements become more stringent.